Nanostructure diffraction gratings for integrated spectroscopy and sensing

ABSTRACT

The present disclosure pertains to metal or dielectric nanostructures of the subwavelength scale within the grating lines of optical diffraction gratings. The nanostructures have surface plasmon resonances or non-plasmon optical resonances. A linear photodetector array is used to capture the resonance spectra from one of the diffraction orders. The combined nanostructure super-grating and photodetector array eliminates the use of external optical spectrometers for measuring surface plasmon or optical resonance frequency shift caused by the presence of chemical and biological agents. The nanostructure super-gratings can be used for building integrated surface enhanced Raman scattering (SERS) spectrometers. The nanostructures within the diffraction grating lines enhance Raman scattering signal light while the diffraction grating pattern of the nanostructures diffracts Raman scattering light to different directions of propagation according to their wavelengths. Therefore, the nanostructure super-gratings allows for the use of a photodetector array to capture the surface enhanced Raman scattering spectra.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 13/716,122, entitled “Nanostructure DiffractionGratings for Integrated Spectroscopy and Sensing” and filed on Dec. 15,2012, which is incorporated herein by reference.

GOVERNMENT INTEREST

This invention was made with government support under contractNNX07-AL52A, awarded by the National Aeronautics and SpaceAdministration (NASA), and contract NSF-0814103, awarded by the NationalScience Foundation (NSF). The Government has certain rights in theinvention.

RELATED ART

Spectroscopy and most spectroscopy based chemical sensing techniquesrely on spectrometers to perform spectral measurement of opticalradiations. One type of optical spectrometer relies on diffractiongratings to separate spectral frequency components of radiations.Diffraction gratings often comprise periodically arranged metal ordielectric lines on transparent substrates which serve as supportingmaterials. Optical radiations of different frequencies can be spatiallyseparated and measured by using photodetector arrays.

The advancement of nanotechnology has created a new class of chemicaland biological sensors that rely on the resonance shift of metal anddielectric nanostructure devices as the sensing transduction mechanism.One well investigated nanostructure optical resonance type is localizedsurface plasmon resonance (LSPR), which occurs in metal nanostructures.LSPR is the collective oscillation of free electrons (known as surfaceplasmons) in metal nanostructures. At a certain frequency, the plasmonsresonate with incident light, resulting in strongly enhancedelectromagnetic field near the nanostructure surface. Resonancefrequencies in nanostructures change when chemical or biochemical agentsbind onto on surface of the nanostructure. Traditionally, opticalspectrometers are used to perform the spectral measurements fordetermining the resonance frequency shift. One type of chemical sensorrelies on measurement of the resonance frequency change using opticalspectrometers.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure can be better understood with reference to the followingdrawings. The elements of the drawings are not necessarily to scalerelative to each other, emphasis instead being placed upon clearlyillustrating the principles of the disclosure. Furthermore, likereference numerals designate corresponding parts throughout the severalviews.

FIG. 1 illustrates a subwavelength period nanohole array.

FIG. 2 illustrates a super-period nanohole array diffraction grating.

FIG. 3A illustrates a cross sectional view of a blazed nanostructurediffraction grating.

FIG. 3B illustrates a top view of a blazed nanostructure diffractiongrating.

FIG. 4 illustrates a rectangular nano-aperture array diffractiongrating.

FIG. 5 illustrates a nanogrid array diffraction grating.

FIG. 6 illustrates a super-period nanoslit array grating with a smallnano-grating period and a large diffraction grating period.

FIG. 7 illustrates the calculated spectra of the zero-order transmissionand the first order diffraction from a super-period metal nanoslitgrating.

FIG. 8 shows a SEM picture of a super-period metal nanoslit grating.

FIG. 9 shows a first order diffraction intensity distribution capturedby a coupled charge device (CCD) photodetector array when thesuper-period nanoslits device was exposed to air, methanol and acetone.

FIG. 10A illustrates a measurement of surface plasmon resonance in thesuper-period nanoslit grating from the zeroth-order transmission.

FIG. 10B illustrates a measurement of surface plasmon resonance in thesuper-period nanoslit grating from the first-order diffraction.

FIG. 11 illustrates calculated zeroth order transmittance and firstorder diffraction from a super-period metal nanohole array grating.

FIG. 12 shows a SEM picture of an e-beam patterned super-period nanoholearray grating in a thin gold film.

FIG. 13A illustrates an electric field intensity distribution profile onthe near field plane 20 nm above the metal surface at 750.5 nmwavelength.

FIG. 13B illustrates an electric field intensity distribution profile onthe near field plane 20 nm above the metal surface at 760.5 nmwavelength.

FIG. 14A shows a graph of near electric field intensity versus thewavelength at the top center of an inner nanohole aperture within thesuper-period grating period.

FIG. 14B shows a graph of near electric field intensity versus thewavelength at the top center of an outer nanohole aperture within thesuper-period grating period.

FIG. 15 shows spatially dispersed first order diffraction imagescaptured by a CCD when the super-period metal nanohole array gratingarea was exposed to air, methanol and isopropyl-alcohol.

FIG. 16A is a graph showing measured zeroth order transmission spectrameasured by using a commercial spectrometer.

FIG. 16B is a graph showing measured first order diffraction spectraobtained with the disclosed surface plasmon resonance spectrometer.

FIG. 17 illustrates a traditional surface enhanced Ramen spectrometeroptical system setup.

FIG. 18 illustrates the integrated nanostructure surface enhanced Ramanspectrometer with the disclosed nanostructure diffraction grating.

DETAILED DESCRIPTION

The disclosure described herein is generally directed to nanostructuresin the sub-wavelength scale within the grating lines of diffractiongratings. The nanostructures within the grating lines have designedsurface plasmon resonances for metallic nanostructures or opticalresonances for dielectric nanostructures. The resonance frequenciesshift as the nanostructure surfaces interact with chemical andbiological agents of interest. Measurement of surface plasmon resonanceor optical resonance is accomplished by measuring either the spectrum ofthe reflected light or the spectrum of transmitted light, using opticalspectrometers. The presently disclosed device and method eliminates theuse of external optical spectrometers when measuring the resonance shiftcaused by the presence of chemical and biological agents. The presentdisclosure describes the measurement of resonance frequency and thesimultaneous shift when utilizing the combination of a nanostructurediffraction grating and a photodetector array.

The grating periods of diffraction gratings are larger than thewavelengths to be measured, typically from several to several hundredtimes of the longest wavelength to be measured. According to thediffraction theory, different spectral components of radiation propagateto different directions following the equation:

$\begin{matrix}{{{{Sin}(\theta)} = {m\frac{\lambda}{P}}},{m = 0},{\pm 1},{\pm 2},\ldots} & (1)\end{matrix}$where θ is the angle of the diffraction, m is the order of thediffraction, λ is the wavelength, and P is the diffraction gratingperiod. Equation 1 is relevant to normal light incidence to the gratingsurface. If the incident angle is not normal to the surface of thediffraction grating, the diffracted angle θ must be correctedaccordingly. The diffraction grating lines can be made of metal,dielectric, or other materials. The grating lines can be flat or tiltedsuch as in blazed gratings.

FIG. 1 depicts a subwavelength nanohole array in a thin metal (such asgold or silver) film 12. The enhanced transmission 16 of incident light14 through the subwavelength period nanoholes 12 occurs when thefrequency of incident light 14 is tuned to the resonance frequency ofthe periodic nanohole array 12. The local plasmon resonance in theperiodic nanoholes contributes to the enhanced light transmission 16through the nanohole array 10. Enhanced optical transmission and theunderlined surface plasmon resonance can only be measured in either thetransmission 16 or the reflection 18 because the period of the nanoholes12 in the array 10 is smaller than the wavelengths to be measured.

The resonance frequencies in metal nanostructures 10 are generallymeasured by using an optical spectrometer 20 containing a diffractiongrating 22 to measure the reflection or the transmission from thenanostructure device. Traditional optical diffraction gratings 22 arecomprised of metal or dielectric lines placed in periodic patterns withthe width 13 greater than the wavelength of the light to be measured.The grating lines are arranged periodically on transparent substrates.

In contrast, the present disclosure illustrates nanostructures of thesubwavelength scale within the grating lines of a traditionaldiffraction grating. The nanostructures within the grating lines havesurface plasmon resonances or non-plasmon optical resonances. Theresonance frequencies shift as the nanostructure surfaces interact withchemical and biological agents to be measured. The modified resonancespectra are angularly and spatially separated by the diffractiongrating. The nanostructures have feature sizes smaller than thewavelengths of radiations. These nanostructure diffraction gratingsenhance the capability for spectral measurements and also significantlyreduce the physical dimension size of the sensor instruments.

FIG. 2 illustrates a nanohole array diffraction grating 180. The grating180 comprises a substrate layer 42 composed of a transparent material.As used herein, a transparent material is a material with the physicalproperty of allowing light to pass through the material without beingabsorbed. There are a number of suitable transparent substratematerials, for instance quartz, plastic or glass. It is to be understoodthat other transparent materials may be used as the substrate layer 42of the present disclosure. A grating layer 44 is next deposited on topof the substrate layer 42. In one exemplary embodiment, the gratinglayer 44 comprises of a planar metal, such as gold or silver, or adielectric material. Suitable metal surfaces in accordance with thepresent disclosure include various noble metals, e.g. gold, silver orplatinum, as well as base metals such as copper, aluminum, chromium, andalso various metal alloys, etc. The metal may be provided in the form ofa film, preferably a thin film.

The grating 180 comprises a series of diffraction grating lines 181.Each grating line 181 consists of an array of subwavelength sizednanoholes 182 located within the grating lines 181. Although the grating180 in FIG. 2 illustrates an array of nanoholes, it is to be understoodthat these nanostructures may take the form of any shape, whetherrecessed or bored into the grating surface (i.e., a hole or a trench) orfixed upon and protruding from the grating surface (i.e., a raisedcircle, line or square). By way of example, the nanostructure can havecircular, square, rectangular, elliptical or triangular shapes, providedthat the dimensions of the shape are smaller than the wavelengths to bemeasured. Each diffraction grating line 181 has a finite number ofnanoholes 182 in one dimension and infinite (practically very large)number of nanoholes 182 in another dimension. The nanohole array gratinglines 181 are arranged periodically with a diffraction grating period of186. The diffraction grating period 186 is larger than the wavelengthsbeing measured, typically several times of the longest wavelength to bemeasured. This produces angularly dispersed diffractions. Differentwavelengths are diffracted to different directions following thediffraction grating theory described by equation 1 for the normalincidence of light. In contrast, the width of nanoholes 182 is sizedsmaller than the wavelength of the incident light, thus producing asurface plasmon resonance. Diffraction orders are generated when lightis incident to the grating 180. The diffracted radiations are angularlydispersed in space due to the diffraction, i.e. spectral components ofdifferent wavelengths propagate to different directions in space. Aphotodetector array is used to capture the spectral components ofdiffracted light.

FIGS. 3A and 3B illustrate an additional embodiment of a nanostructurediffraction grating of the present disclosure. FIG. 3A shows across-sectional view of a blazed diffraction grating 300. The blazedgrating 300 has constant line spacing or period 302 which determines themagnitude of the wavelength dispersion caused by the grating. Thegrating lines 304 possess a triangular, sawtooth-shaped cross section,thus forming a step structure. The steps are tilted at the so called“blaze angle” with respect to the grating surface. Subwavelength sizednanostructures 306 are located onto the upper surface of the blazedgrating. FIG. 3B shows a top view illustration of the distribution ofnanostructures 306. The blaze grating period 302 is larger than thewavelengths being measured, typically several times of the longestwavelength to be measured. This produces angularly disperseddiffractions. Different wavelengths are diffracted to differentdirections following the diffraction grating theory. In contrast,nanostructures within the blazed grating lines 306 are sized smallerthan the wavelength of the incident light, thus producing surfaceplasmon or non-plasmon resonance. Diffraction orders are generated whenlight is incident to the grating 300. The diffracted radiations areangularly dispersed in space due to the diffraction, i.e. spectralcomponents of different wavelengths propagate to different directions inspace. A photodetector array can be used to measure the spectralcomponents of diffracted light.

In the embodiment described above in reference to FIGS. 2, 3A and 3B thenanostructures, or nanoholes, may be randomly arranged. In an additionalembodiment, the nanostructures are arranged in a repeating, periodicfashion herein referred to as a “period”. Referring again to FIG. 2, thenanoholes 182 may be positioned so as to provide a period 184 which issmaller than the wavelengths of diffracted light to be measured. Asdescribed above, the diffraction grating lines 181 have a period 186which is larger than the wavelength of the diffracted light to bemeasured. In the embodiment illustrated in FIG. 2, the small period 184is arranged within the large period 186. The large grating period 186 isseveral times the size of the small period 184.

FIG. 4 illustrates a two dimensional elongated nanoaperture arraydiffraction grating 200. The nanoapertures 201 are etched into the metalor dielectric film 181 of a thickness 203. In one example, the apertures202 may have rectangular or elliptical shapes. The grating 200 is on anoptically transparent substrate 205. The nanoapertures 201 have asubwavelength period 207 in the y direction and a diffraction gratingperiod 208 in the x direction. The diffraction grating period 208 islarger than the wavelengths of the diffracted light being measured,typically several times of the longest wavelength to be measured. Thediffractions propagate in the x-z plane. Due to the intrinsic angulardispersion of diffractions caused by the diffraction grating with theperiod 208, the resonance in diffraction orders can be measured with aphotodetector array 210. One example of a photodetector array is aone-dimensional linear photodetector array or a two dimensionalphotodetector array (e.g., a charged coupled device (“CCD”)). Thepolarization of the incident light is in the y direction to effectivelyexcite the resonances in the nano-aperture array structure 200. Noadditional optical spectrometers are needed to measure the resonance inthe nanostructure device.

FIG. 5 illustrates an additional embodiment of a nanostructure opticalgrating. This two dimensional nanogrid array grating 320 comprisesraised nanostructures 322 which protrude above the upper surface of thearray 320. In one example, the nanostructures 322 can have rectangularor elliptical shapes. The grating 320 is supported by an opticallytransparent substrate 324. The nanostructures 322 have a subwavelengthperiod 326 in the y direction and a diffraction grating period 328 inthe x direction. The diffraction grating period 328 is larger than thewavelengths of the diffracted light being measured, typically severaltimes of the longest wavelength to be measured. Because the gratingperiod in the y-direction is smaller than the wavelengths, nodiffractions propagate in the y-z plane. The diffractions propagate inthe x-z plane. Due to the intrinsic angular dispersion of diffractionscaused by the diffraction grating with the diffraction grating period328, the optical resonance in the nanostructure device can be measuredin one of the diffraction orders with a photodetector array 330. Oneexample of a photodetector array is a one-dimensional linearphotodetector array or a two dimensional photodetector array (e.g., aCCD). The polarization of the incident light should be in the ydirection to effectively excite the optical resonances in the nanogridarray structure 320. No additional spectrometers are needed to measurethe resonance in the nanostructure grating device.

In an additional embodiment of the present disclosure, FIG. 6illustrates a dual periodic nanoslit array 40. The nanoslits 46 comprisetrenches or grooves etched into the grating surface 44. Nanoslits 46 arearranged periodically with a large period 50 above the wavelength ofinterest. The large period 50, also referred to as a “super-period,” iscreated by periodically removing nanoslits 46 from a periodic nanoslitarray 10. The super-period 50 functions as a diffraction grating periodwhich is larger than the wavelength of interest. Because of thesuper-period 50 structure, surface plasmon radiations from thenanostructure arrays 40 can be measured from the non-zeroth orderdiffractions (i.e., the first order diffraction). Due to the intrinsicangular dispersion of diffractions, the resonance spectrum can bemeasured with a linear photodetector array or CCD 56 without using anexternal optical spectrometer.

The transmission and diffraction from the super-period nanostructure 40can be obtained by solving Maxwell's equations. FIG. 7 illustrates thecalculated spectra of the zero-order transmission and the first orderdiffraction from the super-period gold nanostructure 40 upon the normalincidence with the incident polarization perpendicular to the metalnanoslits 46. The sharp transmission peak 70 at 0.612 micron wavelengthcorresponds to the surface plasmon resonance that causes the enhancedzeroth-order transmission (dashed line) in the device. FIG. 7illustrates that the spectral peak wavelength in the first orderdiffraction spectrum (solid line) is about the same wavelength of thespectral peak of the zeroth order transmission. An additional surfaceplasmon resonance in the device is observed in the zeroth ordertransmission and in the first order diffraction at 0.762 micronwavelength (71). This resonance does not exist in the regular periodgold nanoslit array. It is due to the surface plasmon resonance in thelarge period grating 50. By using a linear photodetector array, theresonances in the nanostructure device can be obtained by measuring thespatially and spectrally dispersed first order diffraction from thenanostructure grating device. The super-period metal nanoslit array 40itself supports localized surface plasmon resonance and performs thespectral analysis simultaneously.

In an additional embodiment, any of the above-described nanostructuresmay be utilized to study molecular binding interactions between freeanalyte molecules in solution and probe molecules which are linked to orimmobilized on to the nanostructure grating surface. This embodimentcontemplates one or more biomolecules attached to the nanostructuregrating surface. For the purposed of this disclosure, “biomolecules”include, but are not limited to, single and double-stranded nucleicacids, oligonucleotides, proteins and protein fragments, amino acids,peptides, antibodies, antigens, viruses, virus fragments, hormones andsmall molecules such as drugs. The biomolecules may be attached directlyto the grating. Alternatively, or in addition, the biomolecules may beattached to the grating via a number of chemical linker and spacermoieties. The attached biomolecules are then exposed to a test samplecontaining additional biomolecules (analytes) in free solution. Bindingof an analyte to an immobilized biomolecule attached to thenanostructure grating 42 will cause changes in the local index ofrefraction, thus changing the resonance frequencies of the local surfaceplasmons. This shift of the local plasmon resonance frequency can beeasily detected by the photodetector array, i.e. the CCD 56. Themeasurement of this optical resonance shift identifies the existence ofchemical or biological agents on the nanostructure surface 40. This is adirect method of detection which avoids the drawbacks of labels. Thiselimination of the need for labeling is important for at least tworeasons. First, it eliminates the need to chemically modify thebiomolecule and the concomitant concern that the label might alter ormodify the biomolecule's activity or behavior. Second, biomolecules incomplex mixtures (such as nuclear extracts) can be studied directlywithout having to purify them and attempt to label them in the mixture.

In an additional embodiment of the disclosure, any of the presentlydisclosed nanoslit arrays may be utilized to observe the time-dependentbinding interaction between two biomolecules. The kinetics of molecularbinding events may be studied by measuring the change of the localplasmon resonance frequency over time. When an analyte with highaffinity to the immobilized biomolecule is introduced, binding eventscan be observed by monitoring the shift of the resonance frequency. Aninitial rapid change of the local plasmon resonance frequency can beobserved as analyte begins to bind to the many available binding sites(i.e., attached biomolecules). While sample analyte is continuallydelivered to the nanostructure grating 42, analyte molecules continue tobind, thus lowering the available number of binding sites (attachedbiomolecules). The shift of the local plasmon and optical resonancefrequency then levels off as the system reached equilibrium.

In an alternate embodiment, the present invention is directed tointegrated surface enhanced Raman scattering spectroscopy (SERS)measurement based on nanostructure metal or dielectric diffractiongratings. The integrated SERS spectrometers rely on patternednanostructure diffraction gratings. The patterned nanostructurediffraction gratings have two functions: (1) the nanostructures withinthe diffraction grating lines enhance the Raman scattering light; (2)the diffraction grating pattern of the patterned nanostructuresdiffracts Raman scattering light to different directions of propagationaccording to their wavelengths. A linear photodetector array is used tocapture the spectra of the Raman scattering light because of the angulardispersion of the diffracted Raman scattering light.

Raman scattering is a spectroscopic technique used to observevibrational, rotational, and other low-frequency modes in molecules. Itrelies on inelastic scattering, or Raman scattering, of monochromaticlight, usually a laser in the visible, infrared, or ultraviolet range.The laser light interacts with molecular vibration modes, otherexcitations in the system, resulting in the energy of the photons beingshifted up or down. The shift in energy gives information about thevibrational modes in the system. Typically, a sample is illuminated witha laser beam. Raman scattering light from the illuminated spot iscollected with a lens and sent to an optical spectrometer. Scatteredlight at the wavelength of the excitation laser is filtered out with arejection optical notch filter, while the rest of the collected light ismeasured by an optical spectrometer.

The presently disclosed SERS spectrometer and the accompanyingnanostructures may be used to identify particular biological andchemical agents. Each molecule has a unique Raman scattering spectralsignature. Measurement of this shift can be used to identify specificmolecules or analytes. Specifically, the nanostructure is utilized in anintegrated surface enhanced Raman scattering spectrometer. Surfaceenhanced Raman scattering spectroscopy is a label free detectiontechnique that reveals molecular spectral “signatures.” SERS is apowerful sensing technique that has many applications in materialanalysis and sensing.

Spontaneous Raman scattering is typically very weak, and as a result themain difficulty of Raman spectroscopy is separating the Raman scatteredlight from the intense Rayleigh scattered laser light. As a result,narrow spectral band rejection optical filters are needed to block theRayleigh scattered light of the same wavelength of the excitation laserbefore the Raman scattering signals are collected by spectrometers.

A traditional optical setup 100 for Raman scattering spectroscopymeasurement is illustrated in FIG. 17. A laser 110 is focused by a focalobjective lens 112. The narrow bandwidth laser 110 is used to excitechemical or biological molecules on a nanostructured metal surface 114to generate surface enhanced Raman scattering light. The nanostructure114 comprises a substrate layer 116 composed of a transparent material,for example glass. The nanostructure surface layer 118 contains metalnanostructures (periodic or random) 120 on the surface.

Referring again to FIG. 17, the Raman scattering light produced by thelaser 110 have Raman spectra which are uniquely determined by themolecules. Raman scattering light is directed through a second lens 122,filtered through an optical rejection notch filter 124, focused with athird lens 126, collected and sent to an optical spectrometer 130. Theoptical rejection filter 124 is used to reject the strong scatteredlight of the same wavelength as the excitation laser light 110. Becauseof the need for the optical rejection filters to remove incident laserlight, current surface enhanced Raman spectrometers are expensive,bulky, and mainly used in laboratory environments.

The presently disclosed integrated SERS spectrometer comprising apatterned nanostructure diffraction grating 152 is schematicallyillustrated in FIG. 18. A narrow line-width laser 150 is incident to thegrating containing patterned metal nanostructures (nanodots, nanoholes,nanoslits, etc.) 152. In one embodiment, the grating 152 is comprised ofmetal structures and produces localized surface plasmon resonances whichenhance Raman scattering. In this embodiment, the grating layer 118comprises of a planar metal, such as gold or silver, or a dielectricmaterial. Suitable metal surfaces in accordance with the presentdisclosure include various noble metals, e.g. gold, silver, platinum, aswell base metals such as copper, aluminum, chromium, and also variousmetal alloys, etc. The metal may be provided in the form of a film,preferably a thin film. In an additional embodiment, the grating 152 ismade from non-metal material which produces local optical resonanceswhich enhance light-matter interactions to produce large Ramanscattering signal. One example of a non-metal dielectric opticalresonance grating 152 is a guided-mode resonance structure that producesstrongly enhanced near optical field which enhances the Raman scatteringsignal. In one embodiment, the nanostructures comprise periodicnanoholes 153. It is to be understood that the use of nanoholes 153 ismerely exemplary and the repeating nanostructures may comprise the formof slits or any other repeating raised shape, such as raised circles orsquares. The nanoholes 153 are arranged in a periodic, repeatingfashion.

The excitation laser 150 excites the localized resonance (localizedsurface plasmon resonance or other localize optical resonance) of thenanostructures when the laser frequency is tuned close to the resonancefrequency. The localized optical resonance creates a significantlyenhanced optical field near the surface 155 of the nanostructures 153.The highly confined photons interact with the molecules near thenanostructure surface 154 and cause the enhanced Raman scattering withfrequency shifts accordingly to the structure of any bound molecules.Because of the patterned diffraction grating, non-zeroth orderdiffractions 154, 156, 158, 160 may be produced. The shifted Ramanscattering light 154, 156, 158, 160 propagates in different directionsaccording to their wavelengths and the patterned grating period. Ramanscattering spectra can be captured with a linear photodetector array 162to measure the angularly dispersed diffraction. The excitation laserlight 150 and the Raman scattering light 154, 156, 158, 160 areseparated in different directions in non-zero order diffractions becausethey are at different wavelengths. A linear photodetector array 162 isused to capture the spatially dispersed surface enhanced Ramanscattering signal light 154, 156, 158, 160. No optical rejection filtersare required because the Raman excitation laser 150 propagates into adirection different from the diffractions of the Raman scattering lightpropagation 154, 156, 158, 160. As opposed to traditional Ramanspectrometers, the presently disclosed Raman sensor does not needoptical rejection filters to block the Raman excitation laser.

The spectral resolution of the presently disclosed integrated Ramanspectroscopic sensors is dependent upon the power of the angulardispersion, the size of the photodetector pixel and the distance betweenthe photodetector array 162 and the patterned nanostructure grating 152.The angular dispersion of the patterned nanostructure SERS spectrometermay be derived from the following equation:

$\begin{matrix}{\frac{\mathbb{d}\theta}{\mathbb{d}\lambda} = \frac{1}{\sqrt{P^{2} - \lambda^{2}}}} & (2)\end{matrix}$where P is the diffraction grating period and the λ is the wavelength.The spectral resolution Δλ of the integrated Raman spectrometer iscalculated with the following equation:

$\begin{matrix}{{\Delta\;\lambda} = {\Delta\; x{\frac{P}{d}\lbrack {1 - ( \frac{\lambda}{P} )^{2}} \rbrack}^{\frac{3}{2}}}} & (3)\end{matrix}$where Δx is the size of the photodetector pixels and d is the distancebetween the nanostructure grating and the photodetector array. Thespectral resolution can be very high if the diffraction grating periodis slightly larger than the longest wavelengths to be measured.

The presently disclosed integrated Raman spectrometer sensor may utilizeany of the previously described nanostructure gratings, specificallythose described with reference to FIGS. 2, 3A, 3B, 4, 5, and 6. TheRaman spectrometer sensors may employ arrays with nanostructures formedas nanoholes, nano-dots, nano-apertures or nanoslits with nanostructurefeature sizes smaller than the wavelength of light to be measured. Inaddition, nanostructures may also be formed as slits, dashes or otherinwardly protruding shapes, or as any outwardly projecting structuresfixed to the grating surfaces.

EXPERIMENTAL Nanoslit Structure

Fabrication

A super-period nanoslit grating made in a thin gold film on a quartzwafer surface was fabricated by use of a standard e-beam lithographyprocess. A 2 nm thick chromium adhesion layer and a 60 nm gold filmlayer were sputtered onto a quartz substrate using the magnetron DCsputtering technique. A 200 nm electron beam resist layer was thendeposited on top of the gold film by spin coating. The nanoslit patternwas patterned in the e-beam resist layer using the e-beam lithographyand then developed with an e-beam resist developer. After development,reactive ion etching was utilized to transfer the e-beam resist patternto the gold film, followed by the removal of the e-beam resist. FIG. 8shows the SEM picture of the fabricated super-period nanoslits. Thenanoslit width is 140 nm in the 60 nm thick gold film. The smallnanoslits period is 420 nm while the large period is 2100 nm.

Testing

The nanostructure array was tested with a broadband coherent lightsource. The broadband light source is a super continuum broadband laserwith a spectrum range of 500 nm to 2400 nm wavelength. At normalincidence, the angular dispersion of the first order diffraction ismeasured with a CCD. The polarization of the incident light isperpendicular to the metal nanoslits so that localized surface plasmonresonance can be excited. FIG. 9 shows the angularly dispersed firstorder diffraction optical intensity distribution captured by the CCD,(a) with air on the surface, (b) with methanol liquid on the surface,and (c) with acetone liquid on the surface. The horizontal and verticalnumerical numbers in the figure represent the pixels on the CCD.

The correspondence between the wavelengths and the pixels on the CCDmust first be calculated in order to obtain the surface plasmonresonance spectrum. For the first order diffraction at the normalincidence, the diffraction angle is related with the wavelength (λ) andthe super grating period (P) as

$\begin{matrix}{{\sin(\theta)} = \frac{\lambda}{P}} & (4)\end{matrix}$

By measuring the diffraction angle of a Helium-Neon (HeNe) laser at632.8 nm, the spectrometer setup can be calibrated to find thecorrespondence between the wavelengths and pixels on the CCD. Once thecorrespondence between the wavelengths and the pixels on the CCD isfound, one may plot the surface plasmon resonance measured in the firstorder diffraction vs. the wavelength.

Methanol and acetone, with the refractive index of 1.3284 and 1.3586,respectively, were used to test the integrated surface plasmon chemicalsensor. FIG. 10A shows the zeroth order transmission spectra from thedevice in the air (A), and after application of methanol (B) and acetone(C) onto the device surface. Acetone was applied after the measurementwith methanol was complete and the methanol was completely vaporized.The zeroth order transmission spectra in FIG. 10A were measured using acommercial optical spectrometer. FIG. 10B shows the first orderdiffraction spectra measured with the super-period nanoslits sensor when(1) the device was exposed to the air (A), (2) methanol solution isapplied on the device surface (B), and (3) acetone solution is appliedon the nanoslits surface (C). The arbitrary unit used for the firstorder diffraction signal in FIG. 6B corresponds to the intensity levelsmeasured by the CCD. It can be seen that the surface plasmon resonancein the super-period nanoslits at the wavelength of 0.616 micron can becaptured by the CCD in the first order diffraction. The resonancewavelength shifts from 0.616 micron in the air to 0.637 micron whenmethanol is applied, and shifts to 0.646 micron when acetone is appliedlater. The small difference of resonance wavelengths measured with theexternal optical spectrometer and the integrated spectral sensor iswithin the uncertainty range of the external optical spectrometer. Thespectral resolution of the commercial optical spectrometer used in theexperiment is 2.0 nm. The spectral resolution of the integrated surfaceplasmon sensor is 0.7 nm, calculated from the angular dispersion of thesuper-period grating at 0.615 micron wavelength, the pixel size (5.6micron) on the CCD, and the distance (14.5 mm) between the nanoslitdevice and the CCD.

Significantly, the demonstrated new surface plasmon sensor does not relyon an external optical spectrometer to measure the surface plasmonresonance and the resonance shift. The super-period metal nanoslit arrayitself supports localized surface plasmon resonance and performs thespectral analysis simultaneously.

Nanohole Structure

Fabrication

A super-period nanohole array device was fabricated in a 50 nm thickgold film on a quartz wafer with a standard e-beam lithographypatterning and reactive ion etching process. This device is illustratedin FIG. 2. The device comprises a small nanohole array period p and alarge grating period P. The large grating period P is five times of thesmall period p. The super-period nanoholes have a small period of 420 nmand a super grating period of 2100 nm. The SEM picture of the e-beamlithography patterned super-period metal nanohole grating is shown inFIG. 12. The diameter of the nanoholes in the array is approximately 140nm.

Testing

The zeroth order light transmission and the first order diffraction fromthe device were calculated with the normal light incidence. Thepolarization of the incident light is along the effective nanohole arraygrating lines, which is normal to the direction of diffractions.Calculations were carried out using a finite difference time domain(FDTD) software code. FIG. 11 shows the calculated zero-ordertransmission spectrum (dashed line curve) and the first orderdiffraction spectrum (solid curve) from the super-period nanohole arraydevice. It can be seen that two plasmon resonance modes are excited inthe device. The resonance at the longer wavelength corresponds to thetightly confined surface plasmon mode. The resonance at the shorterwavelength corresponds to the weakly confined surface plasmon mode. FIG.11 illustrates that the zeroth order transmission peak due to thetightly confined plasmon resonance mode is at 760.5 nm wavelength andthe first order diffraction peak due to the same plasmon resonance modeis at 750.5 nm wavelength.

FIG. 11 illustrates that the surface plasmon resonance in thesuper-period nanohole array can be observed in the zeroth ordertransmission and also in the first order diffraction. However, resonancepeak wavelength in the first order diffraction is slightly blue-shiftedfrom the resonance peak wavelength in the zeroth order transmission. Togain understanding on the resonance, the electric field intensitydistributions were calculated on a plane 20 nm above nanohole metalsurface at 750.5 nm and 760.5 nm respectively. The results areillustrated in FIGS. 13A and 13B respectively. The electric field at750.5 nm wavelength is stronger than the electric field at 760.5 nmwavelength in the near field. The electric field intensity versus thewavelength was calculated for the top center location of one of the twoinner nanohole apertures within a super-period unit cell. The result isplotted in FIG. 14A. The strongest field enhancement at this location isat 750.5 nm wavelength. The electric field intensity versus thewavelength was calculated for the top center of one of the two outernanohole apertures within a super-period unit cell. The results areplotted in FIG. 14B. The strongest field enhancement is at 749.5 nmwavelength. The first order diffraction peak wavelength of 750.5 nm isapproximately the same as the near field resonance wavelength, althoughthe near field resonance wavelength slightly varies with the location ofmeasurement.

The red-shift of the zeroth order transmission peak wavelength from thenear field resonance wavelength is due to the interference between thesurface plasmon resonance radiations and the directly transmitted lightthrough the nanohole thin metal film. A significant amount of light cantransmit through a 50 nm gold film. The near field is strong due to thelocalized surface plasmon resonance. Therefore, the near field resonancewavelength is primarily determined by the local surface plasmonresonance. The far field diffractions, either the first order or higherorders, avoid the interference between the surface plasmon radiationsand the transmission near the nanohole structure metal film. Therefore,the resonance in diffractions is directly related to the near fieldresonance.

Measurement

The super-period nanohole array device was measured with a supercontinuum broadband laser source. The excitation light was normallyincident from the substrate with the polarization parallel to thenanohole effective grating lines. A CCD was used to capture theangularly dispersed intensity distribution of the first orderdiffraction from the nanohole grating. FIG. 15 shows the spatiallydispersed first order diffraction intensity distribution when differentliquid chemicals were applied to the device surface. A calibration isneeded to obtain the correspondence between the CCD pixels and thewavelengths and to calculate the first order diffraction spectrum. AHeNe laser of 632.8 nm wavelength was used to calibrate the measurementsetup. The HeNe laser was aligned to propagate in the same direction asthe broadband laser. The pixel that corresponds to 632.8 nm wavelengthon the CCD was first identified. Once the pixel corresponding to 632.8nm wavelength is known, the correspondence between all pixels on the CCDand wavelengths can be obtained by using the diffraction equation

$\begin{matrix}{{\sin(\theta)} = {\frac{x}{\sqrt{d^{2} + x^{2}}} = \frac{\lambda}{P}}} & (5)\end{matrix}$where θ is the first order diffraction angle, x is the distance betweenthe first order diffraction spot and the zeroth order transmission spoton the CCD, d is the distance between the nanohole grating device andthe CCD, P is the super grating period, λ is the free space wavelengthcorresponding to x. The distance d is 14.8 mm in the experiment setup.After the calibration, the first order diffraction can be obtained bynormalizing the CCD signal with the responsivity of the CCDphotodetector.

FIG. 15, line (a) shows the angularly dispersed first order diffractionimage (diffracted along the horizontal axis) captured by the CCD whenthe device is in the air. Line (b) shows the spatially dispersed firstorder diffraction image captured by the CCD when methanol liquid isapplied to the device surface. Line (c) shows the spatially dispersedfirst order diffraction image when isopropyl-alcohol is applied to thedevice surface. The images are the intensity signals captured by theCCD. The indexes of refraction of methanol and IPA liquids are 1.328 and1.375, respectively.

FIG. 16A shows the zeroth order transmission spectra when differentliquid chemicals are applied to the nanohole device surface. Theresonance peak wavelength of the stronger resonance at the longerwavelength in the zeroth order transmission is 790 nm when the deviceexposes to the air (A). The resonance peak wavelength shifts from 790 nmto 804 nm when methanol is applied (B), and shifts again to 814 nm whenisopropyl alcohol (IPA) is applied (C). FIG. 16B shows the first orderdiffraction spectra when different liquid chemicals are applied to thedevice. The spectra in FIG. 16B are measured by the CCD and normalizedto the spectrum of incident broadband light source. The vertical axis inFIG. 16B has an arbitrary unit. When chemicals are applied to the devicesurface, the peak diffraction wavelengths in the first order diffractionare shifted. Tracking the shift of the diffraction peak wavelength ofthe longer wavelength resonance, it is found that the first orderdiffraction peak wavelength shifts from 778 nm in the air to 794 nm inthe methanol (B), and again shifts to 809 nm in the IPA (C).

The surface plasmon resonance spectrometer sensor can measure surfaceplasmon resonance from the spatially dispersed first order diffractionwith a single shot CCD image capture. Surface plasmon resonancespectrometers based on the metal nanostructure gratings can perform thefunctions of surface plasmon resonance sensing and resonance spectralmeasurements simultaneously.

Now, therefore, the following is claimed:
 1. A spectrometer foridentifying at least one substances in a sample comprising: ananostructure diffraction grating having a grating layer that comprisesa plurality of nanostructure lines, wherein the sample is positioned onthe grating layer, and wherein each of the nanostructure lines comprisesat least a plurality of nanostructures extending across the respectivenanostructure line in a direction along a surface of the nanostructurediffraction grating such that a width of the respective nanostructureline in the direction is greater than a width of at least one of thenanostructures in the respective nanostructure line; a light sourcepositioned such that light from the light source is incident on thegrating layer, wherein the nanostructure lines are spaced in thedirection with a periodic pattern having a period which is greater thanthe wavelengths of the light such that the wavelengths are spatiallydispersed by the diffraction grating, and wherein each of the pluralityof nanostructures has a respective width less than the wavelengths ofthe light; and an array of photodetectors for measuring the light fromthe diffraction grating at the spatially dispersed wavelengths, whereinthe spectrometer is configured to identify at least one substance of thesample based on a spectra measured by the array of photodetectors. 2.The spectrometer of claim 1, wherein the nanostructures are etched intoa surface of the grating layer.
 3. The spectrometer of claim 1, whereinthe grating layer comprises a metal.
 4. The spectrometer of claim 3,wherein the metal is gold.
 5. The spectrometer of claim 1, wherein thegrating layer comprises a dielectric material.
 6. The spectrometer ofclaim 1, wherein interaction between surfaces of the nanostructures andthe sample causes a shift in surface plasmon resonance in the surfaces,and wherein the spectrometer is configured to determine a wavelength atwhich the surface plasmon resonance occurs based on the measured lightand to identify the substance based on the determined wavelength for thesurface plasmon resonance.
 7. A method for identifying at least onesubstance in a sample, comprising: directing light such that the lightis incident on a grating layer of nanostructure diffraction grating, thegrating layer comprising a plurality of nanostructure lines that arespaced in a direction with a periodic pattern having a period which isgreater than the wavelengths of the light such that the wavelengths arespatially dispersed by the diffraction grating, each of thenanostructure lines comprising a plurality of nanostructures extendingacross the respective nanostructure line in the direction such that awidth of the respective nanostructure line in the direction is greaterthan a width of at least one of the nanostructures in the respectivenanostructure line, wherein the sample is positioned on the gratinglayer, and wherein each of the plurality of nanostructures has arespective width in the direction less than the wavelengths of thelight; measuring by an array of photodetectors the light from thediffraction grating at the spatially dispersed wavelengths; andidentifying at least one substance of the sample based on the measuring.8. The method of claim 7, wherein the nanostructures are etched into asurface of the grating layer.
 9. The method of claim 7, wherein thegrating layer comprises a metal.
 10. The method of claim 7, wherein themetal is gold.
 11. The method of claim 7, wherein the grating layercomprises a dielectric material.
 12. The method of claim 7, whereininteraction between surfaces of the nanostructures and the sample causesa shift in surface plasmon resonance in the surfaces, and wherein themethod further comprises determining a wavelength at which the surfaceplasmon resonance occurs based on the measuring, wherein the identifyingis based on the determining.